The TrSL1 Prototype

This is a functional prototype for our first solid-state integrated amplifier (code name “Transistor Sloop”). The electronics are fully functional and only require refreshes for manufacturability and reflect any feedback we receive from demonstrations. The enclosure is representative of the final look we are going for. The box is larger than we are targeting for the final product as we need to integrate circuit boards where appropriate. Knob style, placement and marking are in process, and determined by the final electronic design.

Audio nerd overview:

The prototype is an all analog integrated amplifier with 2-way active crossovers driving discrete transistor amplifiers in a dual bi-amp mono-block configuration.

Details:

The amplification stage uses BJTs throughout. The design is tuned to preserve the “warmth” and “musicality” of 60’s through 70’s solid-state HiFi amplifiers without the undesired noise and artifacts.

Each amplifier channel (left or right) includes two amplifiers (one each for woofer and tweeter) on a 4 layer, heave copper PCB with an active crossover front end. The PCB provides noise isolation and a low impedance path to power and ground. This ensures that clean power is readily available at each component. The active crossover includes baffle and phase compensation.

Each bi-amp PCB is powered by a dedicated supply through a 440VA toroidal transformer. The DC is regulated through a custom voltage regulator capable of 400% current overhead and estimated .01% load regulation (actual measurement beyond limits of in house test equipment).

Audio inputs are Balanced Line, RCA line and RCA phono. The prototype Phone PCB is configured for MM cartridges, but is designed to be configured for MM or MC. The input, volume and tone circuits are all active.

There are 4 speaker outputs. Two for each channel with each channel providing separate connections for woofer and tweeter.

Preliminary Specifications:

Amplifiers:

Each amplifier stage (2 per stereo channel in bi-amp configuration) tested to 50 Watts into 8 Ohms. <70 Watts into 8 Ohms and 100 Watts into 4 Ohm capability anticipated.

Distortion: THD+N <0.2% at 50 Watts into 8 Ohms.

Amplifier Frequency response: 4 Hz to 255 kHz

Stereo Separation: >80 dB

Demonstration Speakers:

2-way bookshelf size ~13 Liter vented cabinet

8 Ohms impedance

Nominal power handling 50W RMS

Sensitivity 89 dB 1W/1M

Frequency response 32 Hz to 20 kHz

What does all of this mean?

Integrated amplifier:

This means that the amplifier has both left and right channels along with some stereo controls. In comparison, some high-end amplifiers (mono-block) only power one channel with a single audio input and output. Our amplifier is considered integrated and supplies stereo input and output with some controls, such as input select, volume and tone control.

Dual mono-block:

This requires an explanation of why mono-block amplifiers are offered. A mono-block amplifier allows for each stereo channel to be driven by a dedicated amplifier in a standalone enclosure with it’s own power input. This is done to provide the best possible channel separation and system flexibility. The drawback is that you must also purchase separate front-end components, such as pre-amps, to add controls, like volume and tone, as well as handle inputs from multiple sources.

The prototype is considered a dual mono-block. We include the controls and the two amplifier channels in one enclosure. Each amplification channel is treated as a mono-block as it is a fully separate circuit with a dedicated power supply. The stereo controls are powered from a different power supply than the amplifiers to prevent cross-talk from entering the low power precision circuits.

Active Crossover:

Loud speakers use multiple drivers to reproduce sound across the full audio frequency range. This is because it is difficult to make a single driver that can produce low frequencies and high frequencies well. Woofers tend to be bigger with the ability to move a larger volume of air in order to produce low frequency sound. The mass required makes it inefficient at higher frequencies. Tweeters are very good at moving fast. The have a much lower mass and cannot move a large volume of air.

Since the speaker has multiple elements we want to drive each element with just the frequencies that it can reproduce cleanly and efficiently. Simply sending the full frequency range to all elements will lead to inefficiency and distortion. To deal with this speakers use crossover circuits to separate the incoming audio into the appropriate frequency ranges for each element.

The crossover circuits are usually built into the speakers. This allows for using one amplifier channel (left or right) per speaker and allows you to mix and match amplifiers and speakers. The problem is this requires performing the crossover function in the high power domain. Large passive elements, such as inductors / coils and capacitors which are capable of handling the full power is required. Performing this function at high power, even with the best circuits and components, can lead to artifacts, such as phase shifts between low and high frequencies as well have crossover frequency shifts based on power and temperature.

While 2-way speakers, which are made up of one woofer and one tweeter, are capable of providing a full frequency response in a simpler and more compact system, the frequency of the crossover point between the two elements (typically 2kHz to 3kHz) tends to occur at an important range for most music – voice and guitar. This phase and frequency shifts of the passive crossover can cause the mid-range of the audio to sound “muddy.”

High quality speakers tend to avoid this issue by using more speaker elements, woofer, mid and tweeter, to push the crossover points to less noticeable frequencies. This increases the complexity and cost of the crossover circuits.

The prototype provides a 2-way active crossover in response to the more difficult 2-way speaker configuration. Since we are handling the crossover function in the low power domain, we have the opportunity to provide speaker corrections that are not available in passive crossovers.

The first is baffle compensation. The speaker face, or baffle, is small compared to the lower wavelengths produced by the woofer. This means that less of the low frequency sound gets directed toward the listener vs the higher frequencies. The result is in greater efficiency at higher frequencies. To compensate for this the crossover increases the amplitude of lower frequencies based on the size of the speaker face. The prototype compensation is based on the speaker face size of the speakers used for demonstration. In production, this can be adjusted per order if the customer chooses to use their own speakers.

Speaker delay / phase compensation. In most speakers, the tweeter and woofer faces are referenced to the face. The tweeters tend to be smaller than the woofers, so the origin point of the sound can be offset from the woofer. Some of the highest-end speakers compensate for this be mechanically offsetting the different elements to have the origins at the same distance as the reference element – usually the woofer. With an active crossover we have the opportunity to tightly adjust the timing of the elements so the sound arrives at the same time. This allows for phase compensation to be achieved with more mainstream speaker designs. The prototype phase compensation corrects for the offset between the woofer and tweeter in the demonstration speakers. In production, this can be adjusted per order.

The end result of the active crossover is that we can provide big, superior quality sound in conveniently sized speaker.

Bi-amp:

This means that we have two amplifiers built into the PCB for each stereo channel. The woofer and tweeter outputs have a dedicated amplifier allowing for the use of the active crossover.